4.2.2 1350 ºC mantle potential temperature cases
Model 12: MPT = 1350ºC, SHF = 55 mW/m2, full spreading rate = 1 cm/yr. Widespread denudation and anastomosing detachment faults, with both crustal and sub-continental mantle boudins present. The mechanical oceanic lithosphere is 18 km thick at the rift axis. The rift is very asymmetric. The left flank is wide and exhibits anastomosing faults, moderate magmatic accretion, and small allochthons of continental provenance. The right flank is narrow, with only slight accretion and a large block of continent crust derived from the H-block. Melt fraction reaches 19% at lithospheric rupture and break-up. Serpentinization is more widespread here than in any other case.
Model 4: MPT = 1350ºC, SHF = 65 mW/m2, full spreading rate = 1 cm/yr. Another slightly asymmetric rift, but with a more complex structure than Model 3. There is major doming and exhumation of the mantle, with rolling hinge detachment faults extending both the sub- continental mantle and former asthenosphere. Anastomosing detachments are present in the right flank, but small scale (~2 km across). On the left flank, normal faults which merge into a roof detachment along the Moho deform a sub-continental mantle boudin. Peak melt production reached 20% partial melt. The mechanical oceanic lithosphere is very thick at 19 km. Asthenospheric buoyancy is also large since the active melt lithospheric layer is 20% filled with melt. This may explain the high domal topography observed near the rift axis and the large proportion of recrystallized melt pods in the mechanical oceanic lithosphere.
Model 6: MPT = 1350ºC, SHF = 75 mW/m2, full spreading rate = 1 cm/yr. A moderately asymmetric rift with widespread. The left flank of the rift hosts a 20 km across crustal boudin and anastomosing mantle detachment faults, while the right flank hosts high-angle normal faults which exhumes the melt active mantle into the mechanical layer forming a large mantle core complex. Melt production peaks at 22% partial melt, and melt crystallization is especially concentrated in the shear zones beneath rolling hinge detachment faults. The mechanical oceanic lithosphere is relatively thick at 12 km at the seafloor spreading rift axis. Again, the spreading rates controls the thickness of the mechanical ocean lithosphere.
Model 13: MPT = 1350ºC, SHF = 55 mW/m2, full spreading rate = 2 cm/yr. This case is a slightly asymmetric rift with widespread crustal allocthons. The melt fraction reaches 28% and melt crystallization is lopsided towards the right rift flank. The left flank has extensively anastomosed shear zones in the subcontinental mantle yet has relatively few mantle megamullions. The mechanical oceanic lithosphere is 8 km thick at the seafloor spreading axis.
Model 5: MPT = 1350ºC, SHF = 65 mW/m2, full spreading rate = 2 cm.yr-1. A slightly asymmetric rift based on the wider left flank than right flank. The rift also demonstrates minor doming of the mantle and anastomosing detachment faults on the left flank which are less prominent on the right flank. 31% partial melting is reached. The “bottom-heavy” distribution below the rift flanks indicates that the melt should extrude or intrude the lower continental crust in a more realistic model with melt percolation. At the seafloor spreading axis, the mechanical oceanic lithosphere is 6 km thick, which very thin and likely caused the high initial surface heat flow, MPT and fast spreading rate.
Model 18: MPT = 1350ºC, SHF = 75 mW/m2, full spreading rate = 2 cm/yr. Due to a lack of melt extraction in this formulation of GeoFLAC, deformation has localized into two different rift axes. Accounting for this, this case has a very similar structure as Models 17 and 19: widespread crustal allochthons, lack of exhumed sub-continental mantle, and wide flanks; except that magmatic accretion is elevated and concentrated on the right flank. The mechanical lithosphere is 11 km thick and the melt fraction peaks at 25%.